The present invention relates to a ghost removing system for a television receiver.
According to a recent, remarkable increase of high buildings in city areas, the ghost phenomenon on screens of the television receivers has become a serious problem. However, no broadly effective technique for resolving the problem has been developed, although some ideas have been proposed which are applicable to the resolution of the problem in specific situations.
As is well known, the ghost phenomenon is due to the reflections of electromagnetic waves by tall obstructions such as high buildings or mountains. Thus, a signal received by an antenna includes a desired signal wave and an undesired signal wave; the latter being the desired signal reflected from a building or the like, and being known as a ghost signal. The signal received by the antenna is fed to a tuner in which the signal is converted in frequency to an intermediate frequency (IF) and amplified in a VIF stage.
Since there is a phase difference between the desired wave signal and the ghost signal, the output of the VIF stage includes both the desired signal and the ghost signal which are out of phase. The ghost image on the television screen varies depending upon the intensity of the ghost signal, the time delay between the desired signal and the ghost signal, and the phase difference between the carrier waves of the direct wave, (desired signal carrier), and the indirect wave, (ghost signal carrier).
That is, the ghost signal and the desired signal appear at the same polarity, at the reversed polarity or at an intermediate state therebetween according to the phase difference between the carriers of the desired direct wave and the reflected wave. Furthermore, in a color television system, since the phases of the color subcarrier components of the direct and the reflected waves are also shifted relative to each other, the color of the ghost may differ from that of the direct wave. This will make the ghost phenomenon more conspicuous than that in the monochromatic system.
Three general concepts have previously been proposed for preventing the ghost phenomenon from occurring. They are: (a) an improvement of the directivity of the antenna system, (b) a synchronous detection of the video signal with orthogonal detection axes and (c) an addition or subtraction between a direct VIF signal or image signal and that delayed by a time corresponding to an arrival time difference between the direct signal and the ghost.
The technique (a), above, is concerned with the antenna design per se and no consideration is given to the television receiver itself. Of course, the improvement in antenna directivity is very important for excluding ghost waves. However, an improvement in directivity may be difficult, and even though it can be much improved, it is impossible to apply the improved antenna to locations under various circumferences to eliminate the ghost at these locations.
The techniques (b) and (c) are intended to eliminate the ghost phenomena by processing the image signal or IF signal in the television receiver, respectively.
FIG. 1 shows the principle of the method (b) in vector form. In FIG. 1, a phase difference between the desired wave M.sub.s and the ghost wave G.sub.s is represented by .theta.. In detecting the signal containing the waves M and G, a recently developed synchronous detector is used. Setting a detection axis D normal to the ghost wave G.sub.s, the detection output D' is obtained and thus the ghost wave is eliminated.
FIG. 2 is a schematic block diagram for performing the method in FIG. 1. In FIG. 2, the VIF signal produced in the conventional manner is supplied to a carrier-pickup circuit 1 to derive the carrier from the VIF signal. The carrier is supplied to a variable phase shifter (2) to shift the phase thereof to be orthogonal to the VIF signal carrier of G.sub.s. The VIF signal is multiplied by the phase shifted carrier wave in a multiplier 3 which acts as and is often referred to as a synchronous detector. This method seems to be satisfactory and is practically effective to some extent. However, there is a disadvantage inherent to this method. That is, as is well known the television signal is transmitted in a vestigial side band. Further, since the VIF stage has a band-pass characteristic, the frequency band of the television signal is further narrowed, resulting in substantial single side band signal as shown in FIG. 3, which is to be detected. Accordingly, the ghost component contained in the VIF output includes not only the in-phase component but also the 90.degree. out of phase component. In this case, a component orthogonal to the G.sub.s component, i.e., high frequency component, always exists. It is desirable to show, in vector form, a composite component of the G.sub.s and orthogonal component. However, it is hardly possible to do so because the amplitude of the orthogonal component always varies causing the .theta. to change. Therefore, the orthogonal component is omitted from FIG. 3. When the VIF output is detected with the detection axis D, the orthogonal ghost component will still appears in the detection output. Therefore, even if the detection axis D is most suitably adjusted, it is impossible to completely eliminate the ghost image. Further if the adjustment of the detection axis D is performed by shifting it with respect to the in phase component M.sub.s of the desired wave, the orthogonal component of the desired wave in the detection output will be emphasized causing the quality of reproduced image on the television screen to be degraded. The component orthogonal to the desired wave M.sub.s is also omitted from FIG. 3 for the same reason as mentioned. At any rate, the technique (b) has not been put into actual practice due to the above mentioned disadvantages.
The technique (c) is well known as disclosed in, for example, U.S. Pat. No. 3,482,168 titled "Device for Removal of Interfering signals", issued to Saburo Sasao on Dec. 2, 1969, and also as disclosed in Japanese Patent Publication No. 43-21885, published Sept. 19, 1968, in the name of Saburo Sasao and entitled, "Disturbing Signal Removing Device". There are two basic systems using technique (c). The only difference between the two systems is in the stage in which the addition or subtraction is performed. That is, in the first system the addition or subtraction is performed prior to the video detection while in the second system, it is performed after the detection. The above mentioned Sasao U.S. Patent discloses one example of the latter system. Also, FIG. 5 shows one example of a system according to technique (c), wherein detection, i.e., demodulation of the VIF into the complex video, occurs after delay and addition. FIG. 4 shows an example where detection occurs prior to delay and addition. The primary difference in the circuits is the frequency bands over which they must operate.
In the first system (e.g., FIG. 5), the RF or VIF signal is delayed by a time corresponding to a difference in arrival time between the desired signal and the ghost signal, the phase and polarity of the delayed signal are regulated to make them suitable for addition, and then the addition is performed. This is the most appropriate system in principle. However, a high performance and inexpensive continuously variable delay line required to delay the RF or VIF signal is difficult to obtain, and even if obtainable, the stability of the delay line with respect to the variation of the surrounding conditions must be very high due to the fact that the phase regulation must be performed after a relatively long delay time. For example, when an acoustic surface delay line of lithium-niobium monocrystalline plate material is used to delay the RF or VIF signal by 5 .mu. sec., the delay time may be varied by 0.015 .mu. sec for every temperature variation of 10.degree. C. This corresponds to a phase shift of 317.degree. for a signal frequency of 58.75 MH.sub.z. It should be noted that the temperature dependency of delay time of the LiNb monocrystalline plate is 30 ppm/.degree. C which is one of the best delay lines at present. Therefore, it is very difficult to employ this system in a television receiver.
FIG. 4 shows a block diagram of an example according to the second type system in which a complex video signal is supplied to one input of an adder 4 and also to a variable delay 5. The output of delay means 5 is connected to a gain and polarity control circuit 6, whose output is connected to the other input of the adder 4. In the systems shown in FIGS. 4 and 5, the problem inherent to the method (b) may be resolved. However, the correction of the phase difference between the ghost and the desired wave is impossible and therefore the ghost removal may be possible in only certain situations where the image of the phase difference is adequate to these systems.
However, the system in FIG. 4 provides a possibility of using a continuously variable delay line such as a Bucket-Brigade device (BBD) as the variable delay line 5, and the system in FIG. 5 provides a possibility of using such a delay device as above after the gain and polarity control circuit 6. Therefore, technique (c) is much more practical than technique (b). As to "Bucket-Brigade device" reference should be made to IEEE Journal of Solid-State Circuits Vol. SC-4, No. 3, June 1969, pages 131 to 134, "Bucket-Brigade Electronics--New Possibilities for Delay, Time-Axis Conversion, and Scanning" by F, Sangster et al.